KR20210069813A - Liquefied gas re-gasification system - Google Patents

Abstract

Disclosed is a liquefied gas re-gasification system which is used in re-gasification of liquefied gas and can re-gasify liquefied gas at high efficiency. According to an embodiment of the present invention, the liquefied gas re-gasification system is used in re-gasification of liquefied gas and comprises: a liquefied gas transport line to gasify the liquefied gas to deliver the liquefied gas to a user entity; a heating medium circulation line circulating a mixed refrigerant provided to transfer heat for gasifying the liquefied gas from a heat source and having two or more refrigerants with different boiling points mixed therein; an evaporator installed on the heating medium circulation line, and gasifying the mixed refrigerant by heat exchange with the heat source; a gasifier installed on the heating medium circulation line, and using latent heat and thermal energy of gasified mixed refrigerant to gasify the liquefied gas of the liquefied gas transport line by heat exchange with the mixed refrigerant; and a control unit predicting the composition ratio of the mixed refrigerant based on phase equilibrium representing the relationship between temperature and pressure for various composition ratios of the mixed refrigerant, and controlling the composition of the mixed refrigerant based on the predicted composition ratio of the mixed refrigerant.

Description

Liquefied gas regasification system {LIQUEFIED GAS RE-GASIFICATION SYSTEM}

The present invention relates to a liquefied gas regasification system, and more particularly, by using a mixed refrigerant in which two or more refrigerants having a temperature increase effect due to a difference in boiling point are mixed, the liquefied gas is regasified, and the temperature and It relates to a liquefied gas regasification system for predicting the composition ratio of a mixed refrigerant based on pressure.

As environmental side effects such as ozone layer depletion and global warming are emerging, regulations on the use of refrigerants are gradually being strengthened, and the demand for fuels such as natural gas that emits less environmental pollutants is increasing. In order to supply natural gas to demand, a system for regasifying liquefied natural gas stored in a liquefied state in a liquefied gas storage tank is required. The conventional liquefied gas regasification system mainly uses an evaporative heating medium such as propane to receive heat from seawater and transfer the heat to liquefied natural gas (LNG) to regasify the liquefied natural gas into natural gas. are doing

When designing to vaporize liquefied natural gas by one-time heat exchange treatment using propane, the size of the heat exchanger must be very large, so economical efficiency is lowered, the temperature condition of propane must be controlled in a very limited range, and the pressure in the heat exchanger There is a disadvantage in that it becomes difficult to supply natural gas at the temperature required by the customer when the temperature of the seawater drops or the seawater temperature changes. For this reason, the conventional regasification system for liquefied natural gas is generally divided into a revaporizer for vaporizing the liquefied natural gas and a trim heater for heating the natural gas vaporized by the regasifier.

In the conventional liquefied gas regasification system, propane circulates through a circulation pump, receives heat from seawater, and first exchanges heat with natural gas in the first heat exchanger (trim heater) to cool it, and again receives heat from seawater and vaporizes it. After that, it is liquefied by heat exchange with liquefied natural gas in the second heat exchanger (regasifier). At this time, in the trim heater, heating is performed using the sensible heat of propane in a liquid state.

In the case of such a liquefied gas regasification system, while the amount of heat required for LNG vaporization in the regasifier is larger than the amount of heat required for natural gas heating in the trim heater, the trim heater is heated using the sensible heat of liquid propane. Due to this relationship, a larger refrigerant flow rate is required in the trim heater than in the regasifier. Therefore, since the conventional liquefied gas regasification system needs to circulate a refrigerant of an unnecessarily large flow rate with an excessive pressure difference, there is a problem in that the operating cost increases and the efficiency decreases.

In addition, since the trim heater requires a high-pressure condition to use the refrigerant that is not vaporized and a low-pressure state is required to vaporize the refrigerant supplied to the regasifier afterward, the refrigerant is pressurized to a high pressure by a pump to trim After supply to the heater, a large pressure drop must be applied to the refrigerant that has passed through the trim heater, which is a factor that reduces energy efficiency. In addition, in the case of propane used as a refrigerant, since it is highly flammable, the safety of the system may be reduced.

In the case of a conventional indirect regasification system using propane or glycol-water, when the difference between the temperature of the natural gas and the seawater is not large, there may be restrictions on the use of vaporizing the refrigerant. 1 is a graph showing the temperature change according to the heat flow amount (Heat flow) of a propane refrigerant used in a conventional liquefied gas regasification system. When propane is used as a heating medium between liquefied natural gas (LNG) and seawater (SW) and propane is vaporized by seawater to vaporize liquefied natural gas, a phase change process in which propane is vaporized by heat exchange with seawater In (②), a pressure loss occurs and the temperature of propane decreases. In order to increase the heat exchange efficiency between propane and liquefied natural gas, the minimum temperature difference from the delivery temperature of natural gas (e.g., 8°C) so that the graph of the temperature change of propane in the PH diagram of propane does not overlap with the shaded portion in FIG. (minimum temperature approach) or higher, and lower than the seawater temperature by at least the minimum temperature difference.

However, due to the decrease in the temperature of propane in the phase change process (②) of propane, it becomes difficult to secure conditions that are higher than the minimum temperature difference than the delivery temperature of natural gas and lower than the minimum temperature difference of seawater temperature, and the temperature control range of propane is will decrease As such, if the operable range of the propane refrigerant is narrowed, the system may not operate properly even with small changes in temperature, pressure, flow rate, etc., and the propane does not satisfy the minimum temperature difference condition from the delivery temperature of natural gas, so heat exchange is not performed properly. may not be done Therefore, in the case of the conventional liquefied gas regasification system, it becomes difficult to construct a system that matches the temperature required by the natural gas demander by using the latent heat generated during the phase change of the heat transfer medium under the design temperature condition of seawater.

The present invention provides a liquefied gas regasification system that can regasify liquefied gas with high efficiency using a mixed refrigerant in which two or more refrigerants having a temperature gliding effect are mixed in a phase change process. it is for

In addition, the present invention predicts the composition ratio of the mixed refrigerant based on the temperature and pressure of the mixed refrigerant in a saturated state and controls the composition of the refrigerants based on the predicted composition ratio to increase the regasification efficiency of the liquefied gas and reduce the equipment cost. To provide a gas regasification system.

In addition, the present invention is to provide a liquefied gas regasification system that can simplify the system and reduce operating costs, and can regasify the liquefied gas in an environmentally friendly manner.

The problems to be solved by the present invention are not limited to the problems mentioned above. Other technical problems not mentioned will be clearly understood by those of ordinary skill in the art to which the present invention belongs from the following description.

Liquefied gas regasification system according to an embodiment of the present invention, in the liquefied gas regasification system used for regasification of liquefied gas, a liquefied gas transfer line for vaporizing the liquefied gas and sending it to a consumer; a heating medium circulation line that is provided to transfer heat for vaporizing the liquefied gas from a heat source and circulates a mixed refrigerant in which two or more refrigerants having different boiling points are mixed; an evaporator installed in the heat medium circulation line and vaporizing the mixed refrigerant by heat exchange with the heat source; a vaporizer installed in the heating medium circulation line and vaporizing the liquefied gas of the liquefied gas transfer line by heat exchange with the mixed refrigerant using thermal energy and latent heat of the vaporized mixed refrigerant; and a control unit that predicts the composition ratio of the mixed refrigerant based on a phase equilibrium diagram indicating the relationship between temperature and pressure for various composition ratios of the mixed refrigerant, and controls the composition of the mixed refrigerant based on the predicted composition ratio of the mixed refrigerant include

The liquefied gas regasification system according to an embodiment of the present invention includes: an expansion tank installed at the rear end of the vaporizer in the heat medium circulation line, suppressing a volume change of the mixed refrigerant and accommodating the vaporized mixed refrigerant; a temperature sensor for measuring the temperature of the mixed refrigerant in a saturated state inside or at the rear end of the expansion tank; And it may further include a pressure sensor for measuring the pressure of the mixed refrigerant in a saturated state in the inside or the rear end of the expansion tank. The controller may predict a composition ratio of the mixed refrigerant by comparing the temperature and pressure of the mixed refrigerant in the saturated state with the pressure on the phase equilibrium diagram.

The liquefied gas regasification system according to an embodiment of the present invention is controlled by the control unit, and according to the composition ratio of the mixed refrigerant, a refrigerant replenishing unit for supplying a refrigerant that does not meet a target composition ratio among the refrigerants to the expansion tank further comprises can do. The refrigerant replenishment unit, a plurality of replenishment tanks for storing each of the first refrigerant and the second refrigerant constituting the refrigerants; a plurality of replenishment lines for replenishing the expansion tank with the first refrigerant and the second refrigerant stored in the plurality of replenishment tanks; and a flow control valve or pump installed in each of the plurality of replenishment lines to control the supply flow rate of the refrigerant.

The control unit, the refrigerant that does not reach the target composition ratio until a difference between the pressure of the mixed refrigerant measured by the pressure sensor and a target pressure set according to the temperature of the mixed refrigerant is reduced within a preset tolerance range The refrigerant replenisher may be controlled to replenish the expansion tank.

According to an embodiment of the present invention, a liquefied gas regasification system for regasifying the liquefied gas with high efficiency using a mixed refrigerant mixed with refrigerants having a temperature gliding effect in the phase change process is provided. .

In addition, according to an embodiment of the present invention, the composition ratio of the mixed refrigerant is predicted based on the temperature and pressure of the mixed refrigerant in a saturated state, and the composition of the refrigerants is controlled based on the predicted composition ratio to increase the regasification efficiency of the liquefied gas and equipment cost. can reduce

In addition, according to the embodiment of the present invention, it is possible to simplify the system configuration of the liquefied gas regasification system, reduce operating costs, and regasify the liquefied gas in an environmentally friendly manner.

The effects of the present invention are not limited to the effects described above. Effects not mentioned will be clearly understood by those of ordinary skill in the art to which the present invention pertains from this specification and the accompanying drawings.

1 is a graph showing the temperature change according to the heat flow amount of the propane refrigerant used in the conventional liquefied gas regasification system.
2 is a block diagram of a liquefied gas regasification system according to an embodiment of the present invention.
3 is a graph showing the temperature change according to the heat flow amount of the mixed refrigerant used in the liquefied gas regasification system according to an embodiment of the present invention.
4 is a view showing a safety class classification standard of a refrigerant.
5 is an operation flowchart of a liquefied gas regasification system according to an embodiment of the present invention.
6 to 8 are diagrams illustrating phase balance diagrams for explaining the operation of the liquefied gas regasification system according to an embodiment of the present invention.
9 is a block diagram of a liquefied gas regasification system according to another embodiment of the present invention.

Other advantages and features of the present invention, and a method of achieving them, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and the present invention is only defined by the scope of the claims. Unless defined, all terms (including technical or scientific terms) used herein have the same meaning as commonly accepted by common skill in the prior art to which this invention belongs. A general description of known configurations may be omitted so as not to obscure the gist of the present invention. In the drawings of the present invention, the same reference numerals are used as far as possible for the same or corresponding components. In order to help the understanding of the present invention, some components in the drawings may be shown exaggerated or reduced to some extent.

The terms used in the present application are only used to describe specific embodiments, and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as "comprise", "have" or "have" are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one It should be understood that it does not preclude the possibility of the presence or addition of or more other features or numbers, steps, operations, components, parts, or combinations thereof.

The liquefied gas regasification system according to an embodiment of the present invention vaporizes liquefied gas using a mixed refrigerant in which two or more refrigerants having a difference in boiling point are mixed, so that the temperature decreases in the phase change process in which the mixed refrigerant is vaporized in the evaporator It is possible to continuously rise without it, and it is possible to increase the vaporization efficiency of the liquefied gas.

In addition, the liquefied gas regasification system according to an embodiment of the present invention does not predict the composition ratio of the mixed refrigerant by a gas component analyzer, etc., but predicts the composition ratio of the mixed refrigerant based on the temperature and pressure of the mixed refrigerant in a saturated state. By controlling the composition, it is possible to increase the regasification efficiency of the liquefied gas and reduce the equipment cost.

2 is a block diagram of a liquefied gas regasification system according to an embodiment of the present invention. Referring to FIG. 2 , the liquefied gas regasification system 100 includes a liquefied gas transfer line 110 , a heat medium circulation line 120 , a pump 130 , an evaporator 140 , a pressure control valve 150 , and a vaporizer 160 . ), an expansion tank 170 , a refrigerant replenishment unit 180 , and a control unit 190 .

Liquefied gas regasification system 100 regasifies liquefied gas such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), natural gas (NG), petroleum gas ( It may be provided to supply fuel gas such as Petroleum Gas) to the demand 115 .

In one embodiment, the liquefied gas regasification system 100 may be installed in the hull of the ship. A ship is a floating body equipped with a liquefied gas storage tank, and an offshore platform such as a liquefied natural gas carrier (LNG Carrier), FPSO (Floating Production Storage and Offloading), FSU (Floating and Storage Unit), FSRU (Floating Storage and Regasification Unit), etc. may include.

The liquefied gas transfer line 110 may be provided to receive liquefied gas from a liquefied gas storage tank 111 provided on a ship, vaporize the supplied liquefied gas, and send it to a consumer. In the liquefied gas transfer line 110 , a liquefied gas pump 112 for high-pressure delivery of liquefied gas to the upstream side of the vaporizer 160 based on the flow of liquefied gas and a flow rate control valve 113 for controlling the flow rate of liquefied gas ) is provided, and a flow rate control valve 114 for controlling the output flow rate of vaporized liquefied gas (eg, natural gas or petroleum gas) may be provided on the downstream side of the vaporizer 160 .

The heating medium circulation line 120 may be provided to transfer heat for vaporizing liquefied gas from a heat source such as seawater. A mixed refrigerant in which two or more non-flammable refrigerants are mixed may be circulated in the heating medium circulation line 120 . In an embodiment, the liquefied gas regasification system 100 may be configured as a single closed-loop type heat medium circulation line 120 .

A heater (trim heater) may not be provided between the vaporizer 160 and the consumer 115 . Alternatively, even if a heater is provided between the vaporizer 160 and the consumer 115 , the heat medium circulation line 120 may be designed not to exchange heat with the heater provided between the vaporizer 160 and the consumer 115 .

The mixed refrigerant circulates through the pump 130 , the evaporator 140 , and the vaporizer 160 through the heat medium circulation line 120 . In order that the mixed refrigerant is vaporized in the evaporator 140 and the temperature of the mixed refrigerant is increased in the process of phase change from liquid to gas, the mixed refrigerant is a refrigerant in which two or more different refrigerants having a boiling point difference of at least 20 °C or more are mixed. can be

In an embodiment, the refrigerants of the mixed refrigerant are selected to satisfy the condition that the vapor pressure is 25 barg or less based on a temperature of 50° C., and a mass ratio (composition ratio) of the refrigerants may be set. In an embodiment, the mixed refrigerant may have a mixing ratio of the refrigerants such that the difference between the dew point temperature and the boiling point temperature in the 10 to 25 barg pressure (vapor pressure) range of the mixed refrigerant is 10° C. or more.

The pump 130 is for circulating the mixed refrigerant in the heat medium circulation line 120, and is installed at the rear end of the expansion tank 170 in the heat medium circulation line 120 and the front end of the evaporator 140, and the expansion tank 170 It may be configured to supply the mixed refrigerant in a liquid state supplied from the heat medium through the heat medium circulation line 120 to the evaporator 140 .

The expansion tank 170 is installed between the vaporizer 160 and the pump 130 in the heat medium circulation line 120, and can store the mixed refrigerant liquefied in the heat exchange process with the liquefied gas in the vaporizer 160. have. The expansion tank 170 may serve to offset the volume change of the mixed refrigerant according to the temperature and to accommodate the vaporized mixed refrigerant.

The expansion tank 170 maintains a constant pressure of the heat medium circulation line 120 by absorbing the pressure change of the mixed refrigerant according to the operating conditions, and the mixed refrigerant returned to the expansion tank 170 maintains a set temperature range. It may be operated within a pressure range, and the mixed refrigerant in a gaseous state may not be introduced into the pump 130 . Two or more types of liquefied refrigerants may be stored in the expansion tank 170 . The flow rate of the mixed refrigerant recovered from the vaporizer 160 to the expansion tank 170 may be controlled by a flow control valve (not shown).

The evaporator 140 may be installed in the heat medium circulation line 120 and vaporize the liquid mixed refrigerant supplied from the expansion tank 170 by heat exchange with a heat source. The evaporator 140 may be manufactured to withstand the pressure (vapor pressure) of the mixed refrigerant. In an embodiment, the evaporator 140 may be designed to withstand a vapor pressure of 25 barg of the mixed refrigerant based on a temperature of 50° C. of the mixed refrigerant.

In an embodiment, seawater may be used as the heat source. The heat source may be supplied to the evaporator 140 through the seawater supply line 142 by the seawater pressurization pump 144 . In one embodiment, the inlet temperature of the heat source into the evaporator 140 may be about 14 °C or higher, and the discharge temperature from the evaporator 140 may be about 7-8 °C.

The pressure control valve 150 may be installed in the heat medium circulation line 120 , and may be provided to adjust the pressure (flow rate) of the mixed refrigerant supplied to the vaporizer 160 . The vaporizer 160 is installed in the heating medium circulation line 120 , and may regasify the liquefied gas of the liquefied gas transfer line 110 by using the thermal energy and latent heat of the mixed refrigerant vaporized by the evaporator 140 .

The refrigerant replenisher 180 may supply at least one of the refrigerants to the expansion tank 170 according to a composition ratio of the mixed refrigerant inside or at the rear end of the expansion tank 170 . In the embodiment, the refrigerant replenishment unit 180 is a plurality of replenishment tanks (181, 182) for storing two or more refrigerants, respectively, and a plurality of replenishment tanks (181, 182) Replenish the refrigerant stored in each of the expansion tank (170) It may include a plurality of replenishment lines (183, 184) and a plurality of replenishment lines (183, 184) for each of the plurality of flow control valves (185, 186) for controlling the supply flow rate of the refrigerant. The replenishment lines 183 and 184 may be provided with a pump (not shown) that pressurizes the replenishment tank 170 to replenish the refrigerant from the replenishment tanks 181 and 182 .

When the evaporator 140 is made of, for example, a plate type heat exchanger (PHE) made of titanium, in order to use titanium with a thickness of 0.6 mm or less as a heat exchange plate, the design pressure (vapor pressure) of the mixed refrigerant is It is preferably 25 barg or less. When a titanium plate having a thickness of 0.7 mm or more is used to increase the design pressure, the cost of equipment increases significantly because the price increases by more than 10 times compared to a plate with a thickness of 0.6 mm.

In the case of the mixed refrigerant, the dew point and the boiling point of the mixed refrigerant change according to the type of the mixed refrigerant and the mixing ratio of the refrigerant to affect the temperature change characteristics when the phase of the mixed refrigerant is changed, as well as in the evaporator 140 . During the phase change, the pressure (vapor pressure) of the mixed refrigerant also changes. Therefore, it is necessary to select the types of refrigerants and set the mixing ratio (weight ratio) so as to realize desirable temperature change characteristics in the phase change section of the mixed refrigerant while minimizing the equipment cost of the evaporator 140 .

In an embodiment of the present invention, in order to increase the temperature of the mixed refrigerant during the phase change of the mixed refrigerant in the evaporator 140, the liquefied gas is regasified by using the mixed refrigerant in which refrigerants having a boiling point difference of 20° C. or more are mixed. can In addition, in the mixed refrigerant, the difference between the dew point temperature and the boiling point temperature of the mixed refrigerant at a pressure of about 10 to 25 barg is at least 10° C., and at the same time, the pressure of the mixed refrigerant is about 25 barg or less. As much as possible, the mixing ratio (weight ratio) of the refrigerants may be set.

The mixed refrigerant may include a first refrigerant having the highest boiling point and a second refrigerant having a boiling point lower than that of the first refrigerant by 20°C or more. From the viewpoint of increasing the effect of increasing the temperature of the mixed refrigerant during the phase change of the mixed refrigerant in the evaporator 140 , the boiling point of the second refrigerant is preferably 30° C. or more lower than the boiling point of the first refrigerant, and the first refrigerant It is more preferable that the boiling point difference between the and the second refrigerant is 50° C. or more.

3 is a graph showing the temperature change according to the heat flow amount of the mixed refrigerant used in the liquefied gas regasification system of a ship according to an embodiment of the present invention. In FIG. 3 , the horizontal axis represents the amount of heat flow of fluids (seawater, LNG, and mixed refrigerant), and the vertical axis represents the temperature of the fluids. The line marked 'SW' is a graph of temperature change according to the heat flow of seawater, and the line marked 'LNG' is a graph of temperature change according to the heat flow of liquefied natural gas.

According to the present embodiment, refrigerants having a large boiling point difference are used as the mixed refrigerant, and in the graph of the temperature change according to the heat flow amount of the mixed refrigerant, the temperature increases when the phase changes from the liquid state to vaporized (temperature gliding) section (A) appears, thereby satisfying the minimum temperature difference (ΔT) condition compared to the temperature of the liquefied natural gas, and it is possible to exchange heat between the mixed refrigerant and the liquefied natural gas.

In the case of a single refrigerant, the temperature is constant or the temperature decreases while the phase change occurs, but when refrigerants having a difference of more than a set value are mixed, the temperature of the mixed refrigerant gradually increases while the phase change of the mixed refrigerant occurs in the evaporator 140 . As it rises, a temperature gliding effect occurs in which the mixed refrigerant is vaporized.

At the same time, use two or more refrigerants with a large boiling point difference so that the amount of temperature rise during phase change of the mixed refrigerant is 2 to 3 ℃ or more, preferably 10 ℃ or more, and at the same time, the mixing ratio of the refrigerant having a low boiling point is set above a certain level ( 5% by weight or more based on the mixed refrigerant) is preferred.

Therefore, it is possible to implement a regasification system that satisfies the condition that the minimum temperature difference between natural gas and the mixed refrigerant is ensured, and without the need to install a trim heater at the rear end of the vaporizer, the thermal energy and latent heat of the mixed refrigerant can be fully utilized. The liquefied gas can be vaporized efficiently.

In addition, efficient heat transfer to liquefied gas is possible by using the latent heat of the mixed refrigerant, and the liquefied gas regasification system can be simplified and operating efficiency can be increased. It can be more secure than a system that uses media.

The conventional liquefied gas regasification system is operated as a revaporizer and a trim heater, and in order to operate the refrigerant in a liquid state in the trim heater, it must be operated at a high pressure to prevent vaporization of the refrigerant. There is a large difference in the operating pressure of the refrigerant between the and the trim heater.

However, according to this embodiment, the pressure difference in the circulation loop of the heating medium circulation line 120 is small, and since the refrigerant circulates only in a single heat exchange loop, it is enough to pressurize only the pressure consumed in circulation and the head loss to recover liquefied gas. Energy consumption for fire can be reduced.

As such, according to the present embodiment, it is possible to maximize the effect of using the latent heat of the mixed refrigerant to improve the liquefied gas vaporization performance, to reduce the amount of refrigerant required for the liquefied gas regasification system, and to install a trim heater. It is possible to vaporize the liquefied gas even with one vaporizer without the need to simplify the system configuration.

In addition, it is possible to regasify the liquefied gas by a single heat exchange process without undergoing two-stage heat exchange, and it is possible to reduce the energy and pipe size required for the pump by reducing the circulating flow rate of the mixed refrigerant, thereby reducing system equipment cost and process/operation The cost can also be reduced.

In addition, according to an embodiment of the present invention, it is possible to implement a liquefied gas regasification system that efficiently uses the latent heat of the mixed refrigerant while changing the phase of the mixed refrigerant even in a low seawater design temperature range. When an evaporative mixed refrigerant is used, since latent heat is much greater than sensible heat, the flow rate of the circulating mixed refrigerant can be reduced, and thus energy required for circulation of the mixed refrigerant is also reduced.

In order to control the pressure of the mixed refrigerant to be below the design pressure of the evaporator 140 and at the same time obtain a temperature increase effect when the phase change of the mixed refrigerant in the evaporator 140, the lowest boiling point among the refrigerants of the mixed refrigerant The refrigerant may be mixed with other refrigerants to have a weight ratio of 0.05 to 0.30 based on the weight of the mixed refrigerant.

The mixed refrigerant is non-flammable, and at the same time has an ozone depletion potential of 0 and may be composed of refrigerants having a global warming potential of less than 1650. 4 is a view showing a safety class classification standard of a refrigerant. As shown in FIG. 4 , the safety class of the refrigerant is classified according to toxicity and flammability.

In an embodiment, the refrigerants of the mixed refrigerant may be selected from refrigerants having a safety class of A1, except for refrigerants having a safety group of B1 to B3 or A2 to A3. That is, the mixed refrigerant may be formulated in consideration of only refrigerants having a safety rating having low toxicity and low flammability. In addition, the refrigerants of the mixed refrigerant have an ozone depletion potential (ODP) of 0 and a global warming potential (GWP) of less than 1650. It is preferable to be selected from eco-friendly refrigerants.

In an embodiment, the refrigerant having the highest boiling point among the refrigerants of the mixed refrigerant may be contained in a weight ratio of 5 to 30% based on the weight of the mixed refrigerant. When the refrigerant having the highest boiling point is contained in an amount of less than 5% by weight, it is difficult to obtain an effect of increasing the temperature during phase change of the mixed refrigerant due to the difference in boiling point.

Conversely, when the refrigerant having the highest boiling point exceeds 30% by weight, the pressure of the mixed refrigerant in the evaporator 140 may exceed 25 barg due to the high content of the refrigerant having high volatility, and a high pressure of 25 barg or more There may be a problem in that the design thickness of the evaporator 140 must be increased in order to be able to withstand it.

In an embodiment, the mixed refrigerant is R32 (difuloromethane, boiling point -52°C), R134a (1,1,1,2-tetrafluoroethane, boiling point -26°C), R227ea (1,1,1,2,3,3,3) -heptafluoropropane, boiling point -16℃), R125(1,1,1,2,2-pentafluoroethane, boiling point -48℃), R600(isobutane, boiling point -12℃), R601(n-pentane, boiling point 36℃), R601a (isopentane, boiling point 28 °C), R1234yf (2,3,3,3-tetrafluoropropene, boiling point -30 °C) and R1234ze(E) (trans-1,3,3,-tetrafluroro-1-propene, boiling point -19 ℃) may include two or more refrigerants selected from, but is not limited thereto.

In an embodiment, among the refrigerants of the mixed refrigerant, the first refrigerant is any one of R32 (boiling point -52 °C) and R125 (boiling point -48 °C), and the second refrigerant is R227ea (boiling point - 16 °C), R601 (boiling point) 36° C.), R1234ze (E) (boiling point -19° C.), R134a (boiling point -26° C.), and R601a (boiling point 28° C.) may include, but is not limited thereto.

Table 1 shows examples of mixed refrigerants that can be used in the liquefied gas regasification system according to an embodiment of the present invention. The mixed refrigerant may be a fluorocarbon refrigerant (HFC) or a hydrofluoroolefin refrigerant (HFO), and is class A1 according to the American Society of Heating Refrigeration and Air-conditioning Engineers (ASHRAE) safety rating, non-flammable, and ozone depleting. The index may be 0, and the global warming potential may be less than 1650.

In an embodiment, the liquefied gas regasification system includes a temperature/pressure meter (not shown) for measuring the temperature and pressure of the mixed refrigerant in the expansion tank 170 or the temperature and pressure of the mixed refrigerant supplied from the expansion tank 170 . can be provided For example, when the operating temperature and pressure of the mixed refrigerant are out of the set range, the composition ratio of the refrigerants may be adjusted by the refrigerant replenishing unit 180 .

The refrigerant replenishment unit 180 is based on the temperature/pressure measurement value of the mixed refrigerant analyzed by the control unit 190, the optimum refrigerant mixing ratio that maximizes the regasification efficiency according to various process conditions such as the temperature and pressure of the mixed refrigerant By storing the information in advance, the refrigerants can be supplied to the expansion tank 170 so as to have an optimal refrigerant mixing ratio according to the process condition of the mixed refrigerant, the heat source (seawater), and the NG temperature required by the demander.

The controller 180 predicts the composition ratio of the mixed refrigerant based on a phase diagram indicating the relationship between temperature and pressure for various composition ratios of the mixed refrigerant, and determines the composition of the mixed refrigerant based on the predicted composition ratio of the mixed refrigerant. It can be controlled to have a target composition ratio.

The temperature sensor T1 may measure the temperature of the mixed refrigerant in a saturated state in the pipeline at the rear end of the expansion tank 170 . The pressure sensor P1 may measure the pressure of the saturated mixed refrigerant in the pipeline at the rear end of the expansion tank 170 . The controller 190 may estimate the composition ratio of the mixed refrigerant by comparing the temperature and pressure of the saturated mixed refrigerant at the rear end of the expansion tank 170 with the phase equilibrium.

5 is an operation flowchart of a liquefied gas regasification system according to an embodiment of the present invention. 6 to 8 are diagrams illustrating phase balance diagrams for explaining the operation of the liquefied gas regasification system according to an embodiment of the present invention. 5 to 8 , when the pressure/temperature of the mixed refrigerant at the rear end of the expansion tank 170 is measured, the controller 190 may compare the phase balance of the target composition and the pressure/temperature of the mixed refrigerant (S10, S20). ).

The control unit 190 controls the refrigerant that does not reach the target composition ratio until the difference between the pressure of the mixed refrigerant measured by the pressure sensor P1 and the target pressure set according to the temperature of the mixed refrigerant is reduced within a preset tolerance range. It is possible to control the refrigerant replenishment unit 180 to replenish the expansion tank 170 (S30, S40).

In the liquefied gas regasification system according to an embodiment of the present invention, when the mixed refrigerant is used as an intermediate heating medium, the flow rate of the heating medium is reduced by using a phase change process of the mixed refrigerant to increase the efficiency of the system. At this time, in order to exhibit the performance of the liquefied gas regasification system well, the composition of the mixed refrigerant must be kept constant. During system operation, a small amount of leakage may occur from various devices, and refrigerant may leak due to unusual conditions such as malfunction. Therefore, it is important to predict and control the composition of the mixed refrigerant.

In order to predict the composition ratio of the mixed refrigerant, a component analysis apparatus such as gas chromatography (GC) may be used, but a small change in composition may occur in the process of extracting a sample of the mixed refrigerant to perform the analysis. In addition, since it takes time for component analysis, there is a limitation in that it is difficult to observe the composition in real time.

In order to solve this problem, the liquefied gas regasification system according to an embodiment of the present invention predicts the composition of the circulating mixed refrigerant using the temperature and pressure of the mixed refrigerant at the rear end of the expansion tank 170, and the temperature of the mixed refrigerant The valve is controlled to additionally supply insufficient refrigerant based on the composition ratio predicted by the and pressure.

The mixed refrigerant (heat medium) that has passed through the vaporizer 160 in a gaseous state is liquefied and discharged. At this time, if it is not completely liquefied, the gas may flow into the circulation pump and cause damage. In addition, it is necessary to design the heat exchanger of the vaporizer 160 so that the mixed refrigerant is discharged in a supercooled liquid state from the viewpoint of providing a margin in system design.

6 is a diagram illustrating a phase equilibrium diagram of a mixed refrigerant in which the refrigerant R1 and the refrigerant R2 are mixed. In FIG. 6 , the mixed refrigerant having a composition ratio of the highly volatile refrigerant R1 of 0.15 wt% is indicated by a red line, and the mixed refrigerant having a composition ratio of the refrigerant R1 of 0.35 wt% is indicated by a blue line. While the system is operating, the composition ratio of the highly volatile refrigerant R1 gradually decreases, and the phase equilibrium gradually shifts to the right as shown by the light blue arrow.

When the initial mixed refrigerant composition ratio is R1:R2 = 0.35:0.65, the state of the mixed refrigerant occurs as follows.

- State ①: passing through the evaporator 140 and in a gaseous state at 11

- State ②: liquid state of -20 ℃ supercooled through the vaporizer 160

- State ③: The state changed to a saturated liquid at the corresponding temperature while passing through the expansion tank 170

The liquefied gas regasification system according to an embodiment of the present invention observes the temperature and pressure of the mixed refrigerant in a state ③ at the rear end of the expansion tank 170 . Comparing this temperature and pressure with the phase equilibrium, it is possible to predict the composition ratio of the refrigerants. 7 and 8 , when the sub-cooled liquid mixed refrigerant flows into the expansion tank 170, it changes to a saturated state. That is, the liquid mixed refrigerant discharged from the vaporizer 160 at a specific temperature is changed into a saturated liquid state suitable for the temperature in the expansion tank 170 . Therefore, since the phase diagram varies depending on the composition of the mixed refrigerant, the temperature and pressure of the saturated mixed refrigerant at the rear end of the expansion tank 170 are measured and compared with the phase balance according to the composition of the mixed refrigerant. The composition of the refrigerant can be estimated inversely.

7 is an exemplary diagram of a phase equilibrium diagram of a mixed refrigerant having a composition ratio of R134a:R744(CO _{2 ) = 0.95:0.05 (mass %).} Referring to FIG. 7 , the mixed refrigerant is discharged from the vaporizer 160 in a sub-cooled state of -30° C./4 barg (① State). The mixed refrigerant discharged from the carburetor 160 flows into the expansion tank 170 to achieve equilibrium, and the pressure drops to a saturation pressure of -30°C/1.2 barg (② State). Therefore, when the temperature and pressure conditions of the mixed refrigerant are measured as -30℃/1.2barg in the process of changing from the supercooled state (① State) to the saturated state (② state), the composition ratio of the current mixed refrigerant is R134a: R744 = 0.95: 0.05 can be predicted.

8 is an exemplary diagram of a phase equilibrium diagram of a mixed refrigerant having a composition ratio of R134a:R744(CO2)=0.9:0.1 (mass %). Referring to FIG. 8 , the mixed refrigerant is discharged from the vaporizer 160 in a sub-cooled state of -30° C./4 barg (① State). The mixed refrigerant discharged from the carburetor 160 is introduced into the expansion tank 170, and the pressure is reduced to -30°C/2.4 barg as equilibrium is achieved (② State). Therefore, when the temperature and pressure conditions of the mixed refrigerant are measured as -30℃/2.4barg in the process of changing from the supercooled state (① State) to the saturated state (② state), the composition ratio of the current mixed refrigerant is R134a: R744 = 0.9: 0.1 can be predicted.

A phase diagram for each composition ratio of the mixed refrigerant may be preset through simulation or experiment. That is, in a state in which a phase diagram and a saturation curve according to the composition ratio of the mixed refrigerant are set in the control system, the mixed refrigerant in a saturated state using a sensor installed at the rear end of the shell expansion tank 170 . It is possible to predict the composition ratio of the mixed refrigerant in real time by measuring the temperature / pressure of the mixture, and by filling the insufficient refrigerant according to the change in the composition of the mixed refrigerant, the mixed refrigerant can always be controlled at a constant composition ratio (target composition ratio).

According to this embodiment, by maintaining the temperature and pressure of the mixed refrigerant constant, it is possible to prevent a decrease in heat exchange efficiency due to a change in the vaporization temperature of the mixed refrigerant. For example, when the pressure of the mixed refrigerant increases, the vaporization temperature rises and when the pressure decreases, the vaporization temperature decreases, and when a difference occurs from the initial design value, the heat exchange efficiency may decrease, but according to this embodiment, By adaptively adjusting the composition ratio of the mixed refrigerant within an allowable range in real time to prevent temperature and pressure changes of the mixed refrigerant, deterioration of heat exchange efficiency due to changes in temperature, pressure, etc. can be prevented.

According to the mixing ratio of the mixed refrigerant, the gradient of the temperature change with respect to the vaporization flow of the liquefied gas is changed. The change in the slope of the mixed refrigerant is closely related to the change in flow rate and affects the operation efficiency. In extreme cases, the process may not operate normally or the liquefied gas regasification system may enter an area where it is not feasible due to the limitations of the performance of the heat exchanger. Therefore, the refrigerant supplement unit 180 simulates in advance the change in the temperature gradient of the mixed refrigerant according to the mixing ratio of the mixed refrigerant, and based on the simulation result, a failure occurs in the regasification system, or the possibility of entering the regasification impossible state When it is determined that there is, control such as maintaining the mixing ratio of the mixed refrigerant within a limited range or adjusting the flow rate of the mixed refrigerant may be performed.

9 is a block diagram of a liquefied gas regasification system according to another embodiment of the present invention. In the description of the embodiment of FIG. 9 , descriptions that overlap with the same or corresponding components as in the above-described embodiment may be omitted. In the embodiment of FIG. 9 , the pressure sensor P2 and the temperature sensor T2 are installed directly in the expansion tank 170 rather than the pipeline at the rear end of the expansion tank 170 , and the mixed refrigerant in a saturated state in the expansion tank 170 . It is different from the above-described embodiment in that it is used for predicting and controlling the composition ratio of the mixed refrigerant based on temperature/pressure.

The above embodiments are presented to help the understanding of the present invention, and do not limit the scope of the present invention, and it should be understood that various modifications are also included in the scope of the present invention. The protection scope of the present invention should be determined by the technical spirit of the claims, and the protection scope of the present invention is not limited to the literal description of the claims itself, but actually extends to the invention of an equivalent scope of technical value. It should be understood that

100: liquefied gas regasification system 110: liquefied gas transfer line
111: liquefied gas storage tank 112: liquefied gas pump
114: flow control valve 115: demand
120: heat medium circulation line 130: pump
140: evaporator 142: seawater supply line
144: seawater pressure pump 150: pressure control valve
160: carburetor 170: expansion tank
180: refrigerant replenishment unit 181, 182: replenishment tank
183, 184: make-up lines 185, 186: flow control valve
190: control unit P1, P2: pressure sensor
T1, T2: temperature sensor

Claims (5)

In the liquefied gas regasification system used for regasification of liquefied gas,
a liquefied gas transfer line for vaporizing the liquefied gas and sending it to a consumer;
a heating medium circulation line provided to transfer heat for vaporizing the liquefied gas from a heat source, in which a mixed refrigerant in which two or more refrigerants having different boiling points are circulated;
an evaporator installed in the heat medium circulation line and vaporizing the mixed refrigerant by heat exchange with the heat source;
a vaporizer installed in the heating medium circulation line and vaporizing the liquefied gas of the liquefied gas transfer line by heat exchange with the mixed refrigerant using thermal energy and latent heat of the vaporized mixed refrigerant; and
A control unit for predicting the composition ratio of the mixed refrigerant based on a phase equilibrium diagram indicating a relationship between temperature and pressure for various composition ratios of the mixed refrigerant, and controlling the composition of the mixed refrigerant based on the predicted composition ratio of the mixed refrigerant liquefied gas regasification system. According to claim 1,
an expansion tank installed at a rear end of the vaporizer in the heating medium circulation line, suppressing a volume change of the mixed refrigerant and accommodating the vaporized mixed refrigerant;
a temperature sensor for measuring the temperature of the mixed refrigerant in a saturated state inside or at the rear end of the expansion tank; and
Further comprising a pressure sensor for measuring the pressure of the mixed refrigerant in a saturated state inside or behind the expansion tank,
The control unit is a liquefied gas regasification system for predicting the composition ratio by comparing the temperature and pressure of the mixed refrigerant in the saturated state with the pressure on the phase equilibrium diagram. According to claim 1,
The liquefied gas regasification system further comprising a refrigerant replenishment unit controlled by the controller and supplying a refrigerant that does not meet a target composition ratio among the refrigerants to the expansion tank according to the composition ratio of the mixed refrigerant. 4. The method of claim 3,
The refrigerant replenishment unit,
A plurality of replenishment tanks for storing the first refrigerant and the second refrigerant constituting the refrigerants, respectively;
a plurality of replenishment lines for replenishing the expansion tank with the first refrigerant and the second refrigerant stored in the plurality of replenishment tanks; and
Liquefied gas regasification system including a flow control valve or a pump installed in each of the plurality of supplemental lines to control the supply flow rate of the refrigerant. 4. The method of claim 3,
The control unit is
The refrigerant that does not meet the target composition ratio is supplied to the expansion tank until the difference between the pressure of the mixed refrigerant measured by the pressure sensor and the target pressure set according to the temperature of the mixed refrigerant is reduced within a preset tolerance range. Liquefied gas regasification system for controlling the refrigerant replenishment unit to replenish.

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